In a mobile radio communications system, information is exchanged between mobile unit and base station by radio signals. Since each base station can communicate with a mobile unit only within its radio coverage (cell), there are typically many base stations within a service area. And to avoid interruption when the mobile unit moves from one cell to another cell (roaming), a method is required to enable non-interrupted communication. This is achieved through the handover technology of the communication.
Methods of implementing handover of a call to or from a mobile unit between cells in a mobile radio communication system affect the overall efficiency and the quality of the service. Therefore, it is important to achieve a fast and reliable handover between cells.
Generally, the mobile unit speed is an important factor to determine whether it is worthwhile implementing handover of a mobile unit between cells. The assessment of vehicle speed can be carried out by measuring directly. However this method is not suitable to cellular radio applications. As an alternative, the vehicle speed may be estimated by monitoring the Doppler shift of a carrier frequency. FIG. 1 shows the conventional method for estimating movement speed of a mobile unit. First, a signal from the mobile unit is obtained in step 101. An envelope of the signal is calculated and the envelope is squared in step 103. According to the result of step 103, a correlation coefficient is calculated in step 105. Then, the method obtains a corresponding Doppler frequency by referring to a corresponding relationship between correlation coefficient and Doppler frequency in step 107. Lastly, the method estimates the movement speed of mobile unit according to the Doppler frequency in step 109.
However, conventional method has the disadvantage of a limited range. The conventional method can not accurately measure the movement speed of mobile unit when the mobile unit moves a low velocity. FIG. 2 shows the measurement results of the prior art method. There are three distinct curves A, B, C respectively representing three mobile units having distinct powers of carrier signal. Obviously, when the three mobile units moves below a specific speed, e.g. 18 km/hr, the curves A, B, C bend down sharply. As such, one correlation coefficient mean value may map two distinct speeds and system can not determine the correct movement speed of the mobile unit.